1. Field of the Invention
The present invention relates to an image processing method capable of repeatedly recording and erasing a high-contrast image at high speed by uniformly recording the image at high density and uniformly erasing the image in a short period of time; and an image processing apparatus which can be suitably used in the image processing method.
2. Description of the Related Art
As a method for recording and erasing an image onto and from a thermoreversible recording medium (hereinafter otherwise referred to as “reversible thermosensitive recording medium”, “recording medium” or “medium”) from a distance or when depressions and protrusions are created on the surface of the thermoreversible recording medium, there has been proposed a method using a noncontact laser (refer to Japanese Patent Application Laid-Open (JP-A) No. 2000-136022). This proposal discloses that noncontact recording is performed utilizing a reversible thermosensitive recording medium as a transport container used in a product distribution line, and that writing is carried out using a laser and erasure is carried out using hot air, warm water, an infrared heater or the like.
Additionally, recording methods each using a laser are disclosed, for example, in Japanese Patent (JP-B) Nos. 3350836 and 3446316 and JP-A Nos. 2002-347272 and 2004-195751.
The technique described in JP-B No. 3350836 is related to a modified image recording and erasing method including placing a photothermal conversion sheet on a thermoreversible recording medium, then irradiating the photothermal conversion sheet with a laser beam, and forming or erasing an image on the thermoreversible recording medium by means of the heat generated. In the specification thereof, it is disclosed that both formation and erasure of an image can be carried out by controlling the irradiation conditions of a laser beam. Specifically, it is disclosed that by controlling at least one of the irradiation time, the irradiation luminosity, the focus and the intensity distribution, it is possible to control the heating temperature in a manner that is divided into a first specific temperature and a second specific temperature of the thermoreversible recording medium, and by changing the cooling rate after heating, it is possible to form and erase an image on the whole surface or partially.
JP-B No. 3446316 describes use of two laser beams and the following methods: a method in which erasure is carried out with one laser beam being used as an elliptical or oval laser beam, and recording is carried out with the other laser beam being used as a circular laser beam; a method in which recording is carried out with the two laser beams being used in combination; and a method in which recording is carried out, with each of the two laser beams being modified and then these modified laser beams being used in combination. According to these methods, use of the two laser beams makes it possible to realize higher density image recording than use of one laser beam does.
Additionally, the technique described in JP-A No. 2002-347272 is related to a method in which at the time of laser recording and erasure, the front and back of one mirror are utilized, and the form of the luminous flux of a laser beam is changed depending upon the optical path difference and the form of the mirror. Thus, the size of an optical spot can be changed and defocusing is made possible with a simple optical system.
Further, JP-A No. 2004-195751 discloses that a residual image after erasure can be removed substantially completely by employing the following conditions: the laser absorption rate of a reversible thermosensitive recording medium in the form of a label is 50% or more; the irradiation energy is 5.0 mJ/mm2 to 15.0 mJ/mm2, and the product of the laser absorption rate and the printing irradiation energy is 3.0 mJ/mm2 to 14.0 mJ/mm2, at the time of printing; and the product of the laser absorption rate and the printing irradiation energy at the time of erasure is 1.1 times to 3.0 times the above-mentioned product.
Meanwhile, as an image erasing method using a laser, JP-A No. 2003-246144, for example, proposes a method in which an image with clear contrast can be recorded onto a highly durable reversible thermosensitive recording medium by erasing the image such that the energy of a laser beam, the irradiation time of the laser beam and the pulse width scanning speed at the time of erasure are 25% to 65% of those at the time of laser recording.
According to the above-mentioned methods, images can be recorded and erased by the lasers; however, since laser control is not taken at the time of recording, there is a problem that local thermal damage arises at places where lines overlap at the time of recording, and there is a problem that the color-developing density decreases when solid images are recorded.
With the intention of solving these problems, methods of controlling printing energy are disclosed in JP-A Nos. 2003-127446 and 2004-345273.
JP-A No. 2003-127446 describes the following: laser irradiation energy is controlled for every written point, and when printing is performed such that recording dots overlap or printing is performed onto a folded material, the amount of energy applied thereto is reduced; also, when linear printing is performed, the amount of energy is reduced at predetermined intervals so as to lessen local thermal damage and thereby to prevent degradation of a reversible thermosensitive recording medium.
Meanwhile, in JP-A No. 2004-345273, an attempt is made to reduce energy at the time of laser writing, by multiplying the irradiation energy by the expression |cos 0.5R|k(0.3<k<4), where R denotes the angle of a varied-angle point. This makes it possible at the time of laser recording to prevent excessive energy from being applied to a part where linear images overlap and thereby to reduce degradation of a medium, or to maintain contrast without reducing energy too much.
Additionally, as a method for preventing decrease in color-developing density, JP-A Nos. 2004-1264 proposes a method in which in order to prevent a previously recorded image from being erased when additional writing is carried out using a laser, the dot arrangement pitch for sub scanning is made two or more times greater than the color-forming radius of a laser beam and less than or equal to the sum of the color-erasing radius and the color-forming radius of the laser beam, thereby preventing decrease in color-developing density and creation of a trace of erasure.
As just described, in the above-mentioned methods, attempts are made to avoid application of excessive thermal energy to thermoreversible recording media, caused by overlapping at the time of laser recording. Also, since the intensity distribution of a laser beam is generally in the form of a Gaussian distribution in which the central part of the laser beam is great in intensity, written lines can be changed in width by adjusting the irradiation power, without needing to change the irradiation distance. However, since the energy of the central part becomes extremely high, excessive energy is applied to a thermoreversible recording medium, and when recording and erasure are repeatedly carried out, the thermoreversible recording medium degrades at portions corresponding to the central part, which is problematic.
As a result of carrying out a series of earnest examinations so as to solve the above-mentioned problems, the present inventors have previously proposed an image processing method and an image processing apparatus, wherein in the intensity distribution of a laser beam in a cross section substantially perpendicular to the proceeding direction of the laser beam, the irradiation intensity of the central part needs to be approximately equal to or less than that of the surrounding part, with the phrase “approximately equal to or less than” denoting 1.05 or less times, and the irradiation intensity of the central part is preferably 1.03 or less times that of the surrounding part, more preferably 1.0 or less time; ideally, the irradiation intensity of the central part is smaller than, namely less than 1.0 time, that of the surrounding part (JP-A No. 2007-69605). Here, for the definitions of the central part and the surrounding part, Paragraph [0021] in JP-A No. 2007-69605 states that “in the intensity distribution of the laser beam in the cross section substantially perpendicular to the proceeding direction of the laser beam, the ‘central part’ denotes a site corresponding to an area sandwiched between the apical portions of two maximum peaks in the shape of inverted convexities, included in a differential curve formed when a curve representing the intensity distribution is differentiated twice; and the ‘surrounding part’ denotes a site corresponding to an area other than the ‘central part’”.
In JP-A No. 2007-69605, since the intensity distribution is provided in which the irradiation intensity of the central part of the laser beam is approximately equal to or less than that of the surrounding part, uniform energy can be applied to a thermoreversible recording medium, and thus the thermoreversible recording medium does not degrade much even when recording and erasure are repeatedly carried out. However, when letters/characters and/or symbols small in size are recorded in order to record a number of letters/characters and/or symbols on a thermoreversible recording medium with such an intensity distribution, it is necessary to narrow the width of written lines so as to recognize and read them, however, written lines can hardly be changed in width by changing the irradiation power, and thin written lines can be hardly obtained, and thus there may be a case where written letters/characters cannot be recognized due to the closely packed written lines; to change the written lines in width, it is necessary to change the spot diameter of the laser beam by changing the irradiation distance. In order to do so, it is necessary to move a laser device or the thermoreversible recording medium.
Moreover, a gas laser such as a CO2 laser easily causes variation in irradiation power; as to a laser beam exhibiting an intensity distribution in the form of a Gaussian distribution in which the central part of the laser beam is great in irradiation intensity, recording is not hindered even when the irradiation power is slightly reduced, because the irradiation intensity of the central part is great enough, whereas as to a laser beam exhibiting an intensity distribution in which the irradiation intensity of the central part of the laser beam is approximately equal to or less than that of the surrounding part, there is a problem that recording cannot take place when the irradiation power is reduced.
Thus, as things stand at present, provision of an image processing method and an image processing apparatus is hoped for, wherein a thermoreversible recording medium can be uniformly heated using a CO2 laser device, excessive energy is not applied to the thermoreversible recording medium, degradation of the thermoreversible recording medium can be reduced when recording and erasure are repeatedly carried out, durability against repeated use can be improved, and written lines can be changed in width by adjusting the irradiation power, without needing to change the irradiation distance.